Un-overload Object fields and pre-parse damage dice (refactor step 3)

Object.Arm carried four meanings in C (o_arm/o_charges/o_goldval plus
ring bonuses); it is now four fields: ArmorClass, Charges, GoldValue,
and Bonus. Stats.Arm becomes Stats.ArmorClass. The Charges()/GoldVal()
accessor pair is gone.

Damage dice strings ("1x4/1x2") are parsed once into DiceSpec — at
table definition for the bestiary and weapon tables, at creation for
items — instead of re-parsed with atoi on every swing as fight.c
roll_em did. ParseDice preserves the C parse semantics exactly,
including the junk-tolerant "%%%x0" bestiary placeholder and the
"000x0" flytrap reset (regression-tested in dice_test.go); the
flytrap's growing grip becomes DiceSpec{{VfHit, 1}}.

Save format bumps to 5.4.4-go3 (field renames and retypes would
silently zero under gob's match-by-name decoding).

No behavior change; full suite green.

game/weapons.go

@@ -123,20 +123,20 @@ func (g *RogueGame) wield() {
 
 // initWeaps is the weapons.c init_dam[] table.
 var initWeaps = [NumWeaponTypes]struct {
-	dam    string     // damage when wielded
-	hrl    string     // damage when thrown
+	dam    DiceSpec   // damage when wielded
+	hrl    DiceSpec   // damage when thrown
 	launch WeaponKind // launching weapon
 	flags  ObjFlags
 }{
-	{"2x4", "1x3", noWeapon, 0},                    // WeaponMace
-	{"3x4", "1x2", noWeapon, 0},                    // Long sword
-	{"1x1", "1x1", noWeapon, 0},                    // WeaponBow
-	{"1x1", "2x3", WeaponBow, Stackable | Missile}, // WeaponArrow
-	{"1x6", "1x4", noWeapon, Missile},              // WeaponDagger
-	{"4x4", "1x2", noWeapon, 0},                    // 2h sword
-	{"1x1", "1x3", noWeapon, Stackable | Missile},  // WeaponDart
-	{"1x2", "2x4", noWeapon, Stackable | Missile},  // Shuriken
-	{"2x3", "1x6", noWeapon, Missile},              // WeaponSpear
+	{dice("2x4"), dice("1x3"), noWeapon, 0},                    // WeaponMace
+	{dice("3x4"), dice("1x2"), noWeapon, 0},                    // Long sword
+	{dice("1x1"), dice("1x1"), noWeapon, 0},                    // WeaponBow
+	{dice("1x1"), dice("2x3"), WeaponBow, Stackable | Missile}, // WeaponArrow
+	{dice("1x6"), dice("1x4"), noWeapon, Missile},              // WeaponDagger
+	{dice("4x4"), dice("1x2"), noWeapon, 0},                    // 2h sword
+	{dice("1x1"), dice("1x3"), noWeapon, Stackable | Missile},  // WeaponDart
+	{dice("1x2"), dice("2x4"), noWeapon, Stackable | Missile},  // Shuriken
+	{dice("2x3"), dice("1x6"), noWeapon, Missile},              // WeaponSpear
 }
 
 // initWeapon sets up a new weapon (weapons.c init_weapon).